The ion plasma electron gun of the present invention is of the same general type as the gun disclosed in U.S. Pat. No. 3,970,892, and U.S. patent application Ser. No. 596,093. As disclosed in the cited patent and patent application, a recent advance in the art of generating high energy electron beams for use, for example, in e-beam excited gas lasers, is the plasma cathode electron gun. In such an electron gun a plasma is produced in a hollow cathode discharge between the hollow cathode surface and an anode grid operated at a relatively low voltage with respect to the cathode. Electrons are extracted from the discharge plasma through the anode grid and a control grid, and these electrons are accelerated to high energies in a plasma-free region between the grids and an accelerating anode which, typically, is a thin foil window maintained at a relatively high voltage with respect to the cathode. Among the advantages of the plasma cathode electron gun are its structural simplicity and ruggedness, high controllability and efficiency, low cost, and suitability for producing large area electron beams.
The electron guns disclosed in U.S. Pat. Nos. 3,970,892 as well as 4,025,818 have beams with electron distribution which is generally peaked in the center and diminished to zero at the edges of the foil windows. The device disclosed and claimed in U.S. patent application Ser. No. 596,093 depicts an advance in the art by providing an improved structure which generates an electron beam with uniform electron distribution across the entire beam as it emerges from the foil window.
In employing prior art electron beam generators, it has been recognized that beam uniformity is essentially independent of beam intensity. The beam current is proportional to the high voltage power supply current. Thus, controlling the dose rate of electrons bombarding a moving web to be irradiated is simply a question of measuring and controlling the current supplied by the high voltage power supply. In the case of wire ion plasma devices such as those disclosed in U.S. Pat. No. 3,970,892, the high voltage power supply current is the sum of incident helium ions and emitted electrons. The ratio of emitted electrons to incident ions, the secondary emission coefficient, is dependent upon surface conditions on the emitter surface. In view of the fact that these conditions appear to be changeable, simply monitoring the high voltage power supply is inadequate for controlling the dose rate of secondary electrons striking the moving web surface.
In further considering wire ion plasma devices, if one excludes the option of mechanically varying the grid between the plasma chamber and the high voltage emitting electrode, the instantaneous beam intensity of secondary electrons can only be varied by changing the intensity of the plasma discharge and thus the helium ion current. It has been found, however, that varying the plasma current over a factor of two up or down shows a significant undesirable change in beam uniformity as the plasma intensity is varied.
It is thus an object of the present invention to provide a device and method for its operation to enable one to vary the secondary electron beam intensity while maintaining uniformity in the secondary electron beam output. It is a further object of the present invention to provide a means for varying the dose rate of secondary electrons transmitted through a wire ion plasma device striking a moving or stationary web of material while maintaining secondary electron beam intensity over the entire surface of said moving or stationary web.